Method and device for processing data packets

ABSTRACT

The invention proposes a method of encoding data packet by encoding type information and size information of said data packet into the same field. The invention also proposes a method of processing data packets received. The data packet comprises a header part and a message part. The header part comprises at least one bit for indicating the type of said data packet, said method comprising a step (101) of obtaining the size information of said data packet based on said at least one bit.

FIELD OF THE INVENTION

The present invention relates to the communication of two devices, more particularly, to processing data packets transmitted from a second device to a first device.

BACKGROUND OF THE INVENTION

The transmission of information from one device to another device occurs in all sorts of technical fields; for example, information is transmitted via data packets from one mobile phone to another, or from a server to a client, etc. Data packets can be transmitted via wire, for example, telephone line, network line, etc. Data packets can also be transmitted wirelessly, for example, using near field communication (NFC) technology, or using satellite communication technology, etc.

A data packet typically comprises a header part and a message part. The header part is used for indicating some basic information related to the data packet, such as: type of the data packet, size information (i.e. length) of the data packet, etc., which could also be understood as “metadata” or “overhead”. The message part is used for carrying the message (i.e. information) that the data packet intends to carry.

In some cases, different types of data packet may have a different size of the message part. If the data packet needs to carry the size information of the data packet, typically, the header part is used to carry this size information. The header part for example has two sections, one for indicating the type of the data packet, and another one for indicating size information of the data packet. As an example, the header part has two bytes, the first byte being for indicating type information, and the second byte for indicating the size of the data packet.

OBJECT AND SUMMARY OF THE INVENTION

However, in some other cases, since the data packet is required to be as small as possible, the header part has a very limited number of bits and it has to be used for indicating the type of the data packet; and there are no additional bits for indicating the size of the data packet.

For example, in a wireless power transmission system, a transmitter is used for transmitting power to a receiver inductively using at least one primary coil (transmission coil) and at least one secondary coil (receiving coil), and the receiver for example needs to send data packets carrying configuration information to the transmitter, so that the transmitter knows the power requirement of the receiver and knows how to configure the relevant parameters of the transmitter before the transmitter can start to transmit power to the receiver.

One approach to transmit these configuration data packets is the so-called load modulation, i.e. the transmitter applies (transmits) power to the receiver and the receiver varies its power consumption in order to encode binary signals i.e. data packets. In such a data transmission approach, the duration required to transmit a data packet increases by about 5.5 ms for each additional byte. Therefore, it is desirable to keep the overhead on a data packet as small as possible. As a consequence, the header part of this configuration data packet is defined so as to comprise only a single byte. This byte is used for indicating the type of data packet. There is no extra byte for indicating the size of the data packet.

From a first aspect of this invention, it is advantageous to propose a data packet carrying size information of said data packet. The data packet comprises a header part and a message part, said header part comprising at least one bit for indicating the type of said data packet, wherein said at least one bit is also associated with size information of said data packet.

From a second aspect of this invention, a method of encoding a data packet having at least one bit for representing both type information and size information of said packet is proposed; an encoder for encoding such a data packet is also proposed; and a device comprising the encoder is proposed. The device encodes the data packet and sends it to another device for communication with said other device.

From a third aspect of this invention, a method of processing data packets received from a second device by a first device is proposed. The data packet comprises a header part and a message part. The header part comprises at least one bit for indicating the type of said data packet. The method comprises a step of obtaining size information of said data packet, based on said at least one bit which is used for indicating the type of the data packet.

It is very useful to have the size information of a data packet. For example, if a transmitter does not know the size of the data packet, the transmitter does not know whether the data packet is complete or not, and cannot locate the different parts of the data packet. Only when the size information of the data packet is obtained, many different actions can be implemented by the first device.

In the context of this invention, since sometimes the parts other than the message part of a data packet are fixed parts defined by a protocol like a standard, the size of the data packet depends on the size of the message part of the data packet. In other words, as long as the size of the message part is known, the size of the data packet can be known and vice versa.

According to an embodiment of the invention, the method may further comprise a step of identifying said message part from said data packet according to the obtained size information.

According to an embodiment of the invention, said data packet further comprises a checksum part indicating a first value for verifying the correctness of the data packet. The checksum part can be located according to the obtained size information. The method further comprises a step of calculating a second value, using the identified message part; and a step of determining whether said data packet is correct or not by comparing said first value with said second value.

If the determining step determines that the data packet is incorrect; the method further comprises a step of discarding said data packet.

According to an embodiment of the invention, said data packet is associated with an unknown packet type indicated by said at least one bit.

Normally, a protocol, e.g. a standard, defines many different data packet types for communicating different kinds of information between two devices. For some reason, e.g. to create the possibility of defining some new message types in the future, in the previous version of the protocol some values of the bits used for indicating the type of the data packet are reserved for the future. How to define and use these reserved values is not known at the time of defining the previous version of the protocol; these reserved values probably will be defined (i.e. associated with a certain packet type) in the future version of the protocol, e.g. in the upgrade version. A newly defined packet which is sent by the new generation device, is considered a type-unknown packet when it is received by the old generation device.

In the context of this invention, if a device receiving a data packet knows what type this packet is, i.e. the header part is associated with a known packet type, this data packet is referred to as “type-known packet”; and if a device receiving a data packet does not know what type this packet is, i.e. the header part of this packet is associated with an unknown type according to the knowledge of this device, this data packet is referred to as a “type-unknown packet”. For example, the header part for indicating the type information is one byte, of which the range from 0 to 180 is defined, and the range from 181 to 256 is reserved for future use: a data packet which has a header byte with a value between 181 and 256 is called a type-unknown packet.

A type-unknown packet may be received for example if a device which sends the packet is a new generation device in which some new packet types are defined, however the device which receives the packet is an old generation device in which these new types defined in the new generation are undefined/reserved in the old generation.

Normally, when the old generation device receives a type-unknown packet, it will discard this packet immediately since nobody has realized that there is a need to know further information of a type-unknown packet. However, the inventor of this invention found that it would be useful to obtain the size information of such a type-unknown packet.

Therefore, according to an embodiment of the invention, a method is disclosed of obtaining size information of a data packet of unknown type. With the size information of such a type-unknown packet, some useful steps can be implemented by the device receiving this type-unknown packet.

According to an embodiment of this invention, the first device corresponds to a transmitter, the second device corresponds to a receiver, the transmitter transmits power to the receiver inductively; and the receiver sends the data packet by modulating the power drawn from said transmitter. If the data packet is associated with an unknown packet type according to the at least one bit, and if the determining step determines that the data packet is incorrect; the method further comprises a step of aborting the transmission of power to the receiver.

From a fourth aspect of this invention, a first device is proposed for processing a data packet received from a second device. The data packet comprises a header part and a message part. The header part comprises at least one bit for indicating the type of said data packet. The first device comprises a first unit for obtaining the size of said data packet, based on said at least one bit.

According to an embodiment of this invention, the first device further comprises a second unit for identifying said message part from said data packet according to the obtained size information.

According to an embodiment of this invention, wherein said data packet further comprises a checksum part indicating a first value, the first device further comprises a third unit for calculating a second value, using said identified message part, and a fourth unit for determining whether said data packet is correct or not by comparing said second value with said first value. The second unit calculates the second value according to an exclusive-or algorithm.

According to an embodiment of this invention, the first device further comprises a fifth unit for discarding the data packet when it is determined that the data packet is incorrect.

According to an embodiment of this invention, wherein the first device corresponds to a transmitter, and the second device corresponds to a receiver, the transmitter transmits power to the receiver inductively; the receiver sends a data packet by modulating the power drawn from said transmitter; and the data packet is associated with an unknown packet type according to said at least one bit. The first device further comprises a sixth unit for aborting the transmission of power when it is determined that the data packet is incorrect.

From a fifth aspect of this invention, a data packet is proposed. The data packet comprises a header part and a message part, the header part comprising at least one bit for indicating the type of said data packet, and at least one bit is also associated with size information of said data packet. The data packet may also comprise a checksum part for verifying whether the data packet is correct or not.

These and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The description is given for the sake of example only, without limiting the scope of the invention. The reference Figures quoted below refer to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein:

FIG. 1 shows a flowchart according to an embodiment of this invention;

FIG. 2 depicts an example of the structure of data packet 200;

FIG. 3 illustrates a block diagram of the first device 300.

The dotted lines in the Figures indicate related steps or blocks that are optional; in some embodiments, these can be omitted.

Throughout the above drawings, like reference numerals will be understood to refer to like, similar or corresponding features or functions.

DETAILED DESCRIPTION

From the first aspect of the invention, a data packet comprising a header part and a message part is proposed. The header part comprises at least one bit for indicating the type of said data packet, and the at least one bit is also associated with the size information of the data packet.

In such a way, the size information of a data packet can be encoded into the limited size of header part of said data packet and obtained by the device receiving that data packet. There are no additional dedicated bits for indicating the size information of the data packet.

An encoder for encoding such a data packet is proposed. The encoder could be implemented by means of hardware or software, or both. For example it can be implemented by a memory stored with instruction code or by a processor or a chip, etc.

A device comprising the aforementioned encoder is proposed, the device comprising a unit for sending the data packet to another device.

The association between the size information and the header part can be implemented by introducing a look-up table stored in the device receiving the data packet. The look-up table lists the bits that are used for indicating the type of data packet and the size information of this data packet-type.

As indicated above, in many cases, since the message part is the only part whose size may be different depending on the type, the size of the message part (also referred to as “length of the message part”) is the only size information that needs to be encoded.

A concept of size classes can be used to indicate the size information, with a size class being a range of packet types which all have the same size. For example, size class 1 means that the message part consists of one byte, and size class 7 means that the message part consists of 7 bytes. Assuming the header part has one byte, this provides for up to 256 size classes, where each size class can have 1 . . . 256 packets (with the constraint that the total number of packets does not exceed 256).

Table 1 shows an example of this implementation. It is to be noted that only column 1 and column 2 of table 1 are needed for implementing this invention. Column 3 is not necessary for the step of association size information to the header part; it is used just for the purpose of illustrating the concept of the invention.

As shown in table 1, the header parts from 0x00 to 0x0F reflect a message length of size class 1, which means that the packet type represented by any header part with a value from 0x00 to 0x0F has a message part that consists of 1 byte. In this way, a device receiving a data packet can obtain the size information of the data packet from table 1.

From table 1, it can be seen that both the defined type and the reserved type have been associated with size information. Therefore, a device receiving a type-unknown packet also can obtain the size information of such a type-unknown packet; and if a new packet type with a 7-byte message part needs to be defined in an upgrade (future) version of a protocol, one can select any of the reserved packet types from the group of size class 7, i.e. the header part of the selected packet type having a 7-byte message part should be in the range of values between 0x52 and 0x57. As a result, although an old version device does not know the newly defined packet type according to the old version look-up table, the old version device can get the size information of this unknown (undefined) packet. For example, for header byte 0x1F, although table 1 shows that it is a “reserved” packet type, the size of its message part can be obtained, which is 2.

This ensures forward compatibility with future releases without changes to the existing packet size and structure.

TABLE 1 Column 1 Column 2 Column 3 Header byte Size class Packet type 0x00 1 Reserved 0x01 1 Signal Strength 0x02 1 End Power Transfer . . . 0x05 1 Charge Status 0x06 1 Reserved . . . 1 Reserved 0x0F 1 Reserved 0x10 2 Reserved . . . 2 Reserved 0x1F 2 Reserved . . . 0x50 Reserved 0x51 7 Identification 0x52 7 Reserved . . . 7 Reserved 0x57 7 Reserved . . . 0xFC 31 Reserved 0xFD 31 Reserved 0xFE 32 Reserved 0xFF 32 Reserved

Since it will require substantial space to store the table that associates size information of the data packet with the header part (such as table 1), in an advantageous implementation, the size information can be calculated from the header part using a simple formula, i.e. associating the size information with the header part via a formula, e.g. the formula is: header/8+1, where the division is an integer division (i.e. discarding any non-zero remainder; which means for a header with one byte for indicating the type of the data packet, the header having 256 different values representing 256 types, that this formula provides for 32 size classes having 8 possible packet types each. For example, if the header is: 0x52, the size information should be: 0x52/8+1=11, which means for example the message part is 11 bytes.

The formula can be different, based on how many size classes are needed and how many packet types are needed for each size class.

Without losing universality, in the following, the device receiving the data packet is referred to as a first device, and the device for sending the data packet is referred to as a second device. The first device and the second device could be any devices as long as the second device can send data to the first device and the first device can receive the data and interpret the data so as to perform the predefined action according to the received data. The methods and the first device for processing data packets are illustrated in detail hereinafter by way of example without limiting the protective scope of this invention.

FIG. 1 shows a flow chart of a method of processing a data packet by a first device according to an embodiment of this invention.

FIG. 2 shows an example of the structure of the data packet 200. The data packet 200 consists of four parts, namely, preamble part 201, header part 202, message part 203 and checksum part 204. The pre-amble part 201 signals the start of a data packet. The header part 202 indicates the type and size of the data packet. The message part 203 carries the message that the second device intends to transmit to the first device. The checksum part 204 is intended to be used to verify whether the data packet 200 is correct or not by means of the first device.

The first part that follows after the preamble 201 is the header part 202. The header part 202 is used to indicate the type of the data packet 200; in the scenario of this invention, the header part is assumed to be small; it has for example only 8 bits (1 byte).

The header part 202 is followed by the message part 203. The message part 203 has multiple bits for carrying the message. As indicated above, for different types of data packet, the size of the message part may be different. Some packet types may only need 1 byte as message part, while other packet types may need much more than 1 byte for carrying a message.

FIG. 3 schematically illustrates a block diagram of a first device 300.

Now, referring to FIGS. 1-3, the first device 300 comprises a memory 307 for storing the data packet 200 which is received from a second device. The received data packet 200 comprises a header part 202 and a message part 203. The header part 202 comprises at least one bit for indicating the type of said data packet, e.g. the header part 202 has 1 byte (b₀, b₁, . . . , b₇) for indicating the type of the data packet 200. The first device 300 comprises a first unit 301 for performing a step 101 of obtaining the size information of said data packet 200 based on b₀˜b₇.

Optionally, the obtaining step 101 can be performed by checking a look-up table associating b₀˜b₇ with the size information of the packet 200. The look-up table should be pre-stored in the first device 300. An example of such a look-up table is column 1 and column 2 of table 1. For example, according to table 1, for the value of 0xFC, the size of the message part is in class 31, which means for example the message part has 31 bytes.

Advantageously, the obtaining step 101 can be performed by calculating the size of the packet, based on the value of the header part 202 according to a predefined formula pre-stored in the first device 300.

Table 2 shows an example of the pre-stored formulas for calculating the size of the packet (size of the message part) according to the value of the header part 202.

TABLE 2 Header Message Size 0x00~0x1F  1 + (header − 0)/32 0x20~0x7F  2 + (header − 32)/16 0x80~0xDF  8 + (header − 128)/8 0xE0~0xFF 20 + (header − 224)/4

As shown in table 2, for header part values from 0x20 to 0x7F, the formula for calculating the size of the message part is (2+(header-32)/16). For example, if the header part 202 is 0x52, the size of the message part of packet 200 should be: 2+(0x52−32)/16=2+(82−32)/16=5, which means that the size of the message part of packet 200 is 5 bytes (40 bits).

Knowing the size information of the data packet 200, it helps to identify (locate) the different parts of the packet.

Advantageously, the first device 300 may comprise a second unit 302 for performing a step 102 of identifying said message part from said data packet according to the obtained size information. As a result, which bits belong to the message part 203 and which bits belong to the checksum part can be determined. In other words, the message part can be located.

When the different parts of the data packet are located, the first device 300 is then able to further perform some actions. For example, knowing which bits belong to the checksum byte, the first device then is able to verify the correctness of the received data packet 200.

Advantageously, the first device 300 comprises a third unit 303 for performing a step 103 of calculating a second value according to the identified message part; a fourth unit 304 for performing a step 104 of determining whether said data packet is correct or not by comparing a first value indicated by the checksum part 204 with the calculated second value.

The checksum part has a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data packet can be checked at any later time by re-computing the checksum value and comparing it with the stored one. If the checksums do not match, the data was almost certainly altered (either intentionally or unintentionally). The procedure that yields the checksum from the data is called a checksum function or checksum algorithm. A good checksum algorithm will yield a different result with high probability when the data is accidentally corrupted; if the checksums match, the data is very likely to be free of accidental errors.

The checksum part 204 comprises for example a single byte indicating a first value for verifying whether the data packet 200 is correct or not, i.e to verify whether the data packet 200 has been received correctly or not after the packet 200 is received by the first device from the second device.

To determine whether the packet 200 is correct, firstly, the first device has to retrieve the first value from the checksum part 204. Since the checksum part 204 follows the message part 203, the checksum byte 204 can be located when the message part 203 is located.

The first value represented by the checksum byte 204 is calculated according to a predefined algorithm by the second device and is added to the packet 200 as the checksum byte before the packet 200 is sent to the first device 300. If the first device 300 calculates the second value according to the same predefined algorithm, the second value should be the same as the first value. If the two checksum values are the same, the packet 200 has been transmitted correctly, i.e the packet 200 is correct; however, if they are not the same, there has been a transmission error during the transmission of the packet.

There are many algorithms for calculating the first and second value, for example, the longitudinal parity check, which breaks the data into “words” with a fixed number n of bits, and then computes the exclusive-or (referred to as XOR) of all those words. The result is appended to the packet as an extra word. To check the integrity of a message, the first device computes the exclusive-or of all its words (header byte, message part, and checksum byte); if the result is not a word with n zeros, the first device knows that a transmission error occurred.

As an example, the algorithm is: header byte XOR 1st message byte XOR 2nd message byte XOR . . . XOR last message byte. Consider for example the byte sequence 0x23 0x10 0x35 0x06 0x45. The packet starts at the first byte of the sequence, i.e. the header byte is 0x23. This means that the size of the message in the packet is 2 bytes according to table 2, i.e. 0x10 0x35. The byte following the message is the checksum, i.e. 0x06, which is calculated as 0x23 XOR 0x10 XOR 0x35. The last byte in the sequence (i.e. 0x45) is not actually part of the packet (and would be ignored by a transmitter).

Alternatively, any other known checksum algorithms could be used.

Advantageously, the first device further comprises a fifth unit 305 for performing a step 105 of discarding said data packet 200 when the fourth unit 304 determines that the data packet 200 is incorrect.

Since the packet 200 has been verified to be incorrect, the transmitter has to discard the data packet and wait for the next data packet, irrespective of whether the received data packet is a type-known packet or a type-unknown packet.

If the header part 202 corresponds to a type-unknown packet, it means the first device 300 does not know which type the data packet 200 is, and therefore the first device 300 cannot make use of this data packet. In this case, the first device has to discard the received data packet 200 no matter whether the packet is correct or incorrect so as to ensure that a first generation of e.g. transmitter would remain compatible with future generations of receivers.

Advantageously, the first device 300 corresponds to a transmitter, the second device corresponds to a receiver. The transmitter comprises one or more primary coils which are coupled with one or more secondary coils of the receiver. The primary coil is applied to an AC power source to generate a magnetic field which will induce a voltage in the secondary coils; in such a way, the transmitter can transmit power to the receiver. When the transmitter applies power to the receiver, the receiver can send data packet 300 by modulating the power drawn from said transmitter, i.e. by varying the power consumed from said transmitter. The first device 300 further comprises a sixth unit 306 for performing a step 106 of aborting the transmission of power when the fourth unit 304 determines that the data packet is incorrect. If the data packet 200 is sent in this way, the incorrectness of the data packet 200 indicates a power transmission problem since the data packet is modulated onto the power signal.

There are numerous ways of implementing functions by means of items of hardware or software, or both. In this respect, the drawings are also very illustrative, each representing only one possible embodiment of the invention. For example, the above-mentioned unit 301, 302, 303, 304, 305, 306 can be implemented by one or a plurality of memories stored with different instruction codes. These units may also be implemented by one or a plurality of printed circuit boards or by one or a plurality of processors.

It should be noted that the above described embodiments are given for describing rather than limiting the invention, and it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art readily understand. Such modifications and variations are considered to be within the scope of the invention and the appended claims. The protection scope of the invention is defined by the accompanying claims. In addition, any of the reference numerals in the claims should not be interpreted as a limitation to the claims. Use of the verb “comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The indefinite article “a” or “an” preceding an element or step does not exclude the presence of a plurality of such elements or steps. 

What is claimed:
 1. A method of processing a data packet comprising a header and a message in a wireless power transmission system having a first device and a second device, wherein the first device includes a memory for storing a look-up table comprising a plurality of header values and associated message size formulas for calculating a size of a received data packet, said method comprising: receiving by the first device from the second device, the data packet, wherein the header includes a header value that indicates whether a type of the data packet is known to the first device; selecting a message size formula for calculating a size of the message from the look-up table based on the header value of the data packet wherein the look-up table comprises Header Value Range Message Length Formula 0x00~0xIF 1 + (Y − 0)/32  0x20~0x7F 2 + (Y − 32)/16 0x80~0xDF 8 + (Y − 128)/8 0xE0~0xFF 20 + (Y − 224)/4 

wherein Y corresponds to the received header value; and calculating the size of the message based on the header value by applying the selected message size formula.
 2. The method as claimed in claim 1, further comprising: identifying, by the first device, a checksum of the data packet indicating a first value; calculating, by the first device, a second value based on the message; and determining, by the first device, whether the data packet is correct by comparing the second value and the first value.
 3. The method as claimed in claim 2, further comprising calculating the second value using an exclusive-or algorithm.
 4. The method as claimed in claim 2 further comprising: discarding the data packet when the data packet is incorrect.
 5. The method as claimed in claim 2, further comprising: inductively transmitting power from the first device to the second device; receiving, by the first device, the data packet sent by the second device by decoding modulation of the wireless power field generated by the first device; and aborting, by the first device, transmission of the power to the second device when the data packet is determined to be incorrect by comparing the second value and the first value.
 6. The method of claim 1, wherein the look-up table associates a plurality of header value ranges with a corresponding plurality of message size formulas.
 7. The method of claim 6, wherein each of the plurality of header value ranges stored in the look-up table have different range lengths than at least one other header value range length of the plurality of header value ranges.
 8. The method of claim 1, wherein each of the message size formulas comprises a first constant and a variable component, and wherein the variable component varies as a function of the value of the received header value.
 9. The method of claim 8, wherein the variable component is calculated as a quotient comprising a dividend and a divisor, and wherein the dividend is equal to the received header value reduced by a second constant.
 10. A first device for processing a data packet received from a second device in a wireless power transmission system; the first device comprising: a memory configured to store a look-up table comprising a plurality of header values and associated message size formulas for calculating a size of a received data packet; and processing circuitry configured to receive the data packet, the data packet comprising a header and a message, wherein the header stores a header value that indicates whether a type of the data, packet is known to the first device, wherein the processing circuitry is configured to select a message size formula for calculating a size of the message from the look-up table based on the header value of the data packet wherein the look-up table comprises: Header Value Range Message Length Formula 0x00~0xIF 1 − (Y + 0)/32  0x20~0x7F 2 + (Y − 32)/16 0x80~0xDF 8 + (Y − 128)/8 0xE0~0xFF 20 + (Y− 224)/4 

wherein V corresponds to the received header value, and wherein the processing circuitry is configured to calculate the size of the message based on the header value by applying the selected message size formula.
 11. The first device as claimed in claim 10, wherein the processing circuitry is further configured to: identify a checksum of the data packet indicating a first value; calculate a second value based on the message; and determine whether the data packet is correct by comparing the second value and the first value.
 12. The first device as claimed in claim 10, wherein the processing circuitry is further configured to discard the data packet when the processing circuitry determines that the data packet is incorrect.
 13. The first device as claimed in claim 12, wherein the processing circuitry is further configured to: receive the data packet sent by the second device by decoding a modulation of an electromagnetic field generated by the first device; and abort transmitting the power when the processing circuitry determines that the data packet is incorrect.
 14. The first device of claim 10, wherein the look-up table associates a plurality of header value ranges with a corresponding plurality of message size formulas.
 15. The first device of claim 14, wherein each of the plurality of header value ranges stored in the look-up table have different range lengths than at least one other header value range length of the plurality of header value ranges.
 16. The first device of claim 10, wherein each of the message size formulas comprises a first constant and a variable component, and wherein the variable component varies as a function of the value of the received header value.
 17. The first device of claim 16, wherein the variable component is calculated as a quotient comprising a dividend and a divisor, and wherein the dividend is equal to the received header value reduced by a second constant. 